Such machinery is already known and essentially consists of a hydraulic shield and cutting equipment housed within the shield. The cutting equipment as a rule consists of a miller supported by an arm which is suspended from the high part of the shield, and this equipment makes it possible to dig and to break up the soil ahead of the shield, this soil being evacuated rearward by a conveyor means; the hydraulic shield includes a thrust ring which is linked to a hydraulic thrust system and in this manner the shield can be moved forward after the suitable front work on the soil has been done; to achieve this progressive advance, the thrust ring rests on the pipes or hoops already in place behind the machine and the hydraulic thrust system linked at its other end to the shield exerts an advancing force on this shield.
Such machines have long been well known to the experts but incur several drawbacks which hitherto have defied remedy.
In the first place, it is quite difficult to make curves using this type of machinery, which requires earth work both extraneous and of large cross-section and hence much personnel; futhermore, the curves that are made evince very large radii (the radii of curvature generally are hundred-fold or more the drift diameter). Also, in some loose terrains, it is sometimes impossible to achieve curves using the known machinery and the experts must resort to wholly manually performed earthworks. Moreover, even when operating in a straight line, these machines are ill suited to dig in loose terrains because these tend to "pack" the miller means and to draw very high power.
It is the object of the present invention to remedy the drawbacks in the above known machines by providing an improved machine designed to operate on the same general principle as the known ones.
One essential object in particular of the invention is to make possible much shaper curves without thereby having to perform earthwork outside the proper contours; especially the object of the invention is to make curves of which the radii can be easily adjusted down to a minimum value about 15 times the diameter of the work.
Another object of the invention is to provide a machine suitable to operate in any terrain and in particular in a loose one, both in straight lines and in curves.
Thus one object of the invention is to provide in this manner a universal machine which without difficulties can dig galleries in terrains with strata of different kinds.